Solution based processing is a low cost technique that may advantageously be used for electronics fabrication, for example by printing. A wide range of inorganic and mostly organic materials have been developed in the form of inks that can be used as a precursor for printing electronics. One of the major technological challenges however is the formation of semiconductor layers with good electronic properties via such inks and printing techniques. The fabrication of semiconductor quality inorganic films via ink and printing techniques is very challenging due to defects and impurities that can be introduced into the material during the ink formation and film fabrication.
An interesting group of inorganic semiconductor materials for printing electronics are metal chalcogenides, composed of one or more metal atoms from Group 1 to Group 15 of the periodic table, including Lanthanides and Actinides, with one or more Group 16 elements (S, Se, Te). Chalcogenide semiconductors have interesting optical and electronic properties. Their band gap and electronic properties can be tuned by elemental substitution and compositional adjustment. The high tolerance against compositional variation and defects, together with a good electronic mobility, make chalcogenide semiconductors an interesting candidate for printing electronic applications. Two dimensional electronic conduction was recently discovered in some chalcogenides (MoS2, WS2), bringing more attention to chalcogenide materials as a novel family of electronic materials.
Chalcogenide materials have a complicated and unique chemistry, due to the different nature of chemical bonds in metal chalcogenides compared to most other salts. Metal chalcogenides have a covalent structure which gives them unique physical and chemical properties. For example, unlike most salts (such as metal halides) which are quite soluble in aqueous media and many other polar solvents, metal chalcogenides are insoluble in aqueous media and typical organic solvents. Therefore, due to the unique and complicated chemistry of chalcogenide materials, solution processing of such materials is challenging.
Different approaches have been developed for the formation of liquid sources of chalcogenide materials (inks), which can be divided in two general routes: the nanoparticle route and the solution route. The nanoparticle route is based on dispersing of chalcogenide nanoparticle compounds in a liquid carrier. The solution route is based on dissolving metal salts, chalcogenide compounds or constituent elements and chalcogen elements (S, Se, Te) at the molecular scale in a liquid carrier.
The applicability of these techniques is typically evaluated based on the quality of the resulting chalcogenide layer, and the potential for up-scaling of the process from a lab-scale to an industrial scale.
The main challenge in the solution routes for processing chalcogenide materials is in finding a suitable solvent and conditions for dissolving the chalcogenide compounds and especially the chalcogen elements (S, Se, Te).
For example, it is known that chalcogens such as Se can be dissolved in hydrazine (N2H4). However, since hydrazine is a very hazardous chemical (very toxic and explosive), applying this method requires many safety precautions, which limits the up-scaled use of this method.
In “Study of the dissolution behavior of selenium and tellurium in different solvents—a novel route to Se, Te tubular bulk single crystals”, Jun Lu et al, J. Mater. Chem., 2002, 12, 2755-2761, the formation of homogenous solutions of Se and Te in ethylenediamine (C2N2H8) is reported. This process however requires the use of an autoclave and heating to 180° C. It is a disadvantage of this approach that at such temperature the ethylenediamine has a high toxicity, flammability and corrosivity.
Another approach is for example described in US 2011/0014377. A method is proposed for preparing a selenium ink comprising Se stably dissolved in a liquid medium, wherein the selenium ink is hydrazine free and hydrazinium free. In this method, selenium is dissolved in an amine solvent by adding a traceless reducing agent such as for example ammonium formate (NH4CHO2) and formic acid (CH2O2). This method is quite simple and useful for the formation of selenium ink and the coating of metal chalcogenides. However, it requires the use of additives such as ammonium formate and formic acid to trigger the dissolution of selenium in amine solvents. This introduces oxygen atoms into the solution which can be detrimental for chalcogenide semiconductors. Moreover, the decomposition and removal of chemical additives might cause organic reactions and degassing, complicating the post-coating heat treatment process. Therefore, more simple chalcogen and chalcogenide solutions with less additives are preferred.